Process for preparing superconducting niobium-gallium alloy

ABSTRACT

AN IMPROVED METHOD FOR PREPARING SUPERCONDUCTING NOBIUM-GALLIUM ALLOY HAVING A HIGH CRITICAL TEMPERATURE WHEREBY NOBIUM-GALLIUM ALLOY IS DEPOSITED FROM ITS HALIDES AT TEMPERATURES OF 640-700*C. INTO A SUBSTRATE COATED WITH A FIRST LAYER OF A B-TUNGSTEN NIOBIUM-GALLIUM ALLOY DEPOSITED AT TEMPERATURES ABOVE 700*C.

July 16, 1974 1.. J. VIELAND L 3,324,032

PROCESS FOR PREPARING SUPER'CONDUCTING NIOBIUM-GALLTUM ALLOY Filed July 5, 1972 K i E United States Patent :.PROCESS FOR PREPARING SUPERCONDUCTING .NIOBIUM-GALLIUM ALLOY Leon .Joseph Vieland, Princeton, and Arnold Wilbert Wicklund,,Hopewell, N.J., assignors to RCA Corporation, New York, N .Y.

7 Filed July 3, 1972, Ser. No. 268,805

" Int. Cl. B44dl/14,1/18

US. Cl. 29-194 9 Claims ABSTRACT OF THE DISCLOSURE An improved method for preparing superconducting niobium-gallium alloy having a high critical temperature ,whereby niobium-gallium alloy is deposited from its halides at temperatures of 640-700 C. onto a substrate coated with a first layer of a S-tungsten niobium-gallium --alloy deposited at temperatures above 700 C.

This invention relates to an improved method for preparing a niobium-gallium superconducting alloy having a high critical temperature, i.e., onsetof superconductivity (hereinafter designated T,,) by chemical vapor deposition. a,

BACKGROUND OF THE INVENTION GLW. Webb et al. have disclosed in Solid State Communications, Vol. 9, pp. 1769-1773, 1971, a superconductive niobium-gallium alloy having a high critical temperature, e.g., about 20 K. or higher, useful in high field magnets. This alloy was prepared by quenching a B-tung- 'sten alloy from the melt at 1850 C. to room temperature 'a'nd'subsequently' annealing the quenched alloy at about 700 'C. or lower for extended periods. It is believed the highest T alloy has a lattice constant of 5.165, conforms to the stoichiometric formula Nb Ga and is stable only at temperatures of 700 C. or below. The above process, while effective, is time consuming and expensive and does not lend itself to practical application.

An attempt was made by Webb et al. to prepare this 'Nb Ga alloy by'a vapor deposition technique at temperatures of about 750-830 C. The highest T for the product obtained by such method was 17 K. and the product had 'a'latticeconstantof about 5.168 to 5.170, which indicates that the product was rich in niobium. Increasing the gallium concentration and lowering the reaction temperature to obtain the desired alloy promotes the deposition of another alloy, conforming to the formula Nb Ga thus useful deposits of the 'stoichiometric Nb Ga alloy could not be obtained by the method of Webb et al.

SUMMARY OF THE INVENTION We have discovered that high T Nb Ga alloy can be prepared by a two-step chemical vapor deposition process whereby a high T Nb Ga alloy is grown at about 640- 700" C. on a layer of a lower T niobium-rich niobiumgallium alloy deposited on a substrate at higher temperatures.

BRIEF DESCRIPTION OF THE DRAWING The Figure is a schematic representation of a reactor in which the process of the invention can be carried out.

DETAIL DESCRIPTION OF THE INVENTION According to the present process, suitable halides of niobium and gallium are thoroughly admixed in a reactor in suitable proportions so as to provide a fi-tungsten alloy when the halides are reduced, exposed to a reducing agent at.a first temperature above 700 0., preferably about 750 C. or higher, thereby depositing on a substrate positioned within the reactor a first layer of a niobium-rich B- 'tungstein niobium-gallium alloy, and then decreasing the temperature to about 700 C. or lower while decreasing the proportion of niobium halide to deposit a second, higher T layer of p-tungsten niobium-gallium alloy thereon.

-By niobium-rich fi-tungsten niobium-gallium alloy herein is meant an alloy which contains more than the stoichiometric amount required for any alloy conforming to the formula Nb Ga.

The term halide herein is meant to include bromides or chlorides but chlorides are readily formed and are preferred. The chlorides can be formed conveniently by passing a chlorinating gas, such as chlorine or hydrogen chloride, over each of the metals separately in the desired proportions at the temperature of reaction in inlet tubes connected to the main reactor. Niobium, which is solid at all reaction temperatures herein, can be employed in the form of a finely divided powder or turnings to assure adequate contact with the chlorinating gas. Gallium is, of course, molten at the present temperatures.

The exact chemical composition of the chlorides of gallium and niobium formed as above is unknown. However, the relative proportion of each can be monitored by regulating the proportion of the total chlorinating stream, hereinafter exemplified by hydrogen chloride, passed over each metal. According to our preferred process, hydrogen chloride is passed over gallium metal at a constant level while varying the amount of hydrogen chloride passed over the niobium source. About of the reactant hydrogen chloride gas is passed over the niobium source in order that the product deposited be only in the fl-tungsten phase. When higher proportions of hydrogen chloride are passed over gallium, increasing amounts of an alloy corresponding to the formula Nb Ga are formed together with the desired B-tungsten phase.

The chlorides, initially rich in niobium, are fed to a suitable reactor in which they are thoroughly admixed. A smallexcess of hydrogen chloride is preferably also added along with the metal chlorides to the reactor to guard against premature deposition or reaction, which would result in variation of the composition of the resultant deposited alloys.

The metal chloride stream is advanced through the reactor conveniently on an inert carrier gas, such as helium, argon and the like, to a reducing zone, which also contains the substrate. Suitably, hydrogen is employed as the reducing agent, preferably preheated to the reaction temperature. The substrate can be any solid substrate, such as metal, ceramic, high temperature glass, quartz and the like, and can have any desired shape, such as a plate or sheet, box, boat, wire and the like.

At reaction temperatures above 700*" C., the initial layer deposited on the substrate is a ,B-tungsten phase, high niobium content alloy having a fairly low T on the order of 1315 K. Deposition of this first layer is continued until a layer of about 3-5 microns in thickness has been deposited on the substrate. This layer provides a base upon which the desired high T Nb Ga alloy can be grown and which inhibits the deposition of undesired Nb Ga When the first layer has been deposited, the temperature in the reaction zone is lowered to about 700 C. or less, suitably to between 640-700" C. but preferably to about 700 C. The niobium concentration is also reduced so as to deposit an alloy conforming more closely to the formula Nb Ga, by limiting the proportion of the reactant hydrogen chloride feed stream to the niobium source to about 80-85% by weight. The layer deposited on the substrate in the second step is a ,B-tungsten phase niobium-gallium alloy having a near stoichiometric composition conforming to the formula Nb Ga, having a high T generally between 19- 20 K. and having a narrow superconducting transition temperature. Deposition of this layer is continued until a layer of the desired thickness has been deposited on the substrate.

The substrate is then removed from the reaction vessel and cooled to room temperature. Preferably, however, it is maintained at about 700 C. while being flushed with an inert gas after the deposition is stopped to remove any hydrogen which may be incorporated into the lattice of the deposited alloy.

Although the above process has been described as a batch-type process, the process of the invention can be adapted to a semi-continuous or a continuous process by providing a reactor with separate heating and reducing zones and means to advance the substrate from one zone to another, as will be readily apparent to one skilled in the art.

The invention will be further illustrated by reference to the Figure in the following examples, but it is to be understood that the invention is not meant to be limited to the details described therein.

Example 1 A quartz reactor fitted with a Y-shaped inlet tube 11 having an inverted opening 12 facing the rear wall 13 of the reactor 10, a hydrogen inlet tube 14, a flush gas inlet tube 1.5 and exhaust gas tube 16 was flushed with helium gas. The temperature of the heated zone 17 was brought to 750 C. A flow of 100 standard cubic centimeters per minute (hereinafter designated s.c.c.m.) of helium as a carrier gas was introduced through both quartz arms 18 and 19 of inlet tube 1.1 and through inlet tube 15. Arm 18 contained niobium turnings 20 and arm 19 con tained a quartz boat 21 filled with gallium. A hydrogen stream at 400 s.c.c.m. was introduced through hydrogen inlet tube 14. Pure, dry hydrogen chloride was fed to the reactor, divided so that 3 s.c.c.m. was introduced through arm 19 and 55 s.c.c.m. through arm- 18. In addition, 25 s.c.c.m. was fed through arm 22 to provide an excess of hydrogen chloride to prevent premature reaction of the metal chlorides. The chlorides Were admixed in inlet tube 1.1. The configuration of the inverted opening 12 of tube 11 ensures thorough mixing of the chlorides, due to the turbulence which results as the chloride stream exits from opening 12 and impinges upon the rear Wall 13 of the reactor 10. The chloride-laden stream was advanced through 1 I were terminated. A helium flow of about 1000 s.c.c.m. was

passed into the reactor 10 through arms 18 and 19 and flush gas inlet tube 1.5 for about 0.5 hr. and the plate 23 cooled to room temperature.

The plate 23 had a second layer about 30 microns thick of a fl-tungsten niobiumgallium alloy having a T of be- The procedure of Example 1 was followed except the second reaction temperature was reduced to 640 C. The resultant second layer of B-tungsten niobium-gallium alloy had a T of between 19.6 and 19.9 K.

We claim:

1. An article of manufacture which comprises a substrate, a first layer of a niobium-rich niobium-gallium alloy having a fl-tungsten phaseand having a critical temperature on thetorder of 13-15 Ktgand: a second; layer thereon of a niobium-gallium allo'yhaving a fl-tun-gsten phase and a high critical temperature aboveabout- 19"? K. which contains about one molecule of gallium' to three molecules of niobium. "I

2. An article according to claim 1 "whe'rein the first layer is from 3-5 microns thickl v '1;

3. A method of preparing a superconducting niobiumgallium alloy having a high critical" temperature which comprises depositing afirstlayer of a niobium-rich alloy of niobium-gallium in the fi-tungsten phase from amixture of halides of niobium and gallium at a temperature above 700 C. on a substrate and depositin-g thereona second alloy layer, diiferent from said firstlayer, from a mixture of halides of niobium and gallium at a temperature between about 640-700 C., said second alloy containing about one atom of gallium to three atoms of niobium.

4. A method of preparing a superconducting niobiumgallium alloy having a high critical temperature which comprises (a) passing hydrogen chloride over a source of niobium and a source of gallium at a temperature of above 700 C. so that the proportion of chlorides of niobium is about 95% by weight of the total chlorides of niobium and gallium produced, f (b) admixing the resultant chlorides of niobiumfa'rid gallium in the vapor phase at about 750 C., (c) depositing a first layer of a niobium-rich niobiumgallium alloy in the fi-tungsten phase on a substrate in a reducing atmosphere to a depth of 3-5 microns, (d) cooling the reaction zone to a temperature within a range between 640 and 700 C., v 5 p (e) adjusting the proportion of niobium chloridesto -85% by weight of the combined niobium chloride and gallium chloride stream, and (f) depositing on the first layer a second layer 'of a niobium-gallium alloy in the fi-tungsten phase having a critical temperature above about 19 K. on the first layer. v 5. A method of preparing a superconducting niobiumlgallium alloy having a high critical temperature which comprises I (a) passing a hydrogen halide over a source of niobium and a source of gallium at a temperature of above 700 C. so that the proportion of halides of niobium is about by weight of the total halides of niobium and gallium produced,

(b) admixing the resultant halides of niobium and gallium in the vapor phase at a temperature of above 700 C.,

(c) depositing a first layer of a niobium rich niobiumgallium alloy in the fi-tungsten phase on a substrate in a reducing atmosphere,

(d) cooling the reaction zone-to a temperature within a range between 640 and 700 C., i

(e) adjusting the proportion of halides of niobium Within the range of about 80-85% by weight of the total halides of niobium and gallium, and

(f) depositing a second alloy layer on the first layer.

6. A method according to claim 5 wherein the temperature in step (a) is about 750 C.

7. A method according to claim 5 wherein the deposition in step (c) is continued until a layer of about 3-5 microns in thickness has been deposited on the substrate.

8. A method according to claim 5 wherein the halides j OTHER REFERENCES are chlorides' Webb et aL, Superconductivity Above 20 K. In Stai- A 8 Whmm an 39 chiometfic N11 02:, in Solid State Communications, 9 20 hydrogen chlonde gas 1s added to the metal chlonde pix 17694773 1971 stream in step (b). 5

CAMERON K. WEIFFENBACH, Primary Examiner References Clted 1 UNITED STATES PATENTS US. Cl. X.R.

3,397,084 8/ 1963 Krieglstein 29-194 29-599, 1l7--l07.2 R, 217

3,425,825 2/1969 Wilhelm 117-107.2 R 1 

